The present invention relates to a mobile radio communication system such as a cellular telephone, operative in both an analog modulation mode and a digital modulation mode.
Recent advancements toward digitization in the field of mobile radio communication systems, such as portable telephones, has been remarkable and automotive vehicle telephones. For this reason, well-known conventional analog modulation systems are gradually being replaced by digital modulation systems, or by dual-mode modulation systems operative in both an analog modulation mode and a digital modulation mode. The dual-mode cellular telephones are advantageous in practical use in view of the fact that the service area available to the digital modulation system is still limited within a specific area.
The components required for the radio transmitter used in such a mobile radio communication, system include a power amplifier for amplifying RF signals of adequate power to be supplied to an antenna. In the dual-mode transmitter, the power amplifier is generally required to have high efficiency in the analog modulation mode, while high linearity is required in addition to high efficiency in the digital modulation mode.
In conventional power amplifiers for transmitters, it is known to switch between two power amplification circuits in accordance with the modulation type required. The simplest arrangement is the provision of two selectable power amplification circuits; one power amplification circuit being dedicated to the analog modulation system and the other power amplification circuit being dedicated to the digital modulation system. Unexamined Japanese patent application No. HEI 5-199127/1993 discloses such a power amplification circuit for a transmitter which includes both a non-linear power amplification circuit and a linear power amplification circuit which may be combined with each other through switches. It is important thing in the analog modulation mode to realize high efficiency; therefore, a non-linear operation region (i.e. in the vicinity of a saturation region) of a non-linear power amplification circuit is preferably utilized. On the other hand, in the digital modulation mode such an arrangement is likely to cause distortions during modulation. To reduce such distortions, a linear operation region of the non-linear power amplification circuit is preferably utilized in a relatively lower output range. Meanwhile, the digital modulation mode, when it is operated in a relatively high power output range, requires the selective use a non-linear power amplification circuit and the linear power amplification circuit in order to realize both high efficiency and high linearity.
However, conventional power amplifiers utilizing the above-described switching technology is cost disadvantageous since it requires two different types of power amplification circuits. In addition, it is further disadvantageous that a significant amount of power loss is realized when each RF signal passes through the switches. Hence, a need exists for a single power amplification circuit which includes the capability of being operated in both the analog modulation mode and the digital modulation mode.
When a single power amplification circuit is operated in both the analog modulation mode and the digital modulation mode, a problem may arise in the power added efficiency, as will be explained in detail below.
The power added efficiency .eta. of the power amplifier is generally defined as follows:
.eta.=(AC Output Power--AC Input Power)/DC Input Power where AC Input Power represents the power of the input RF signal, AC Output Power represents the power of the output RF signal, and DC Input Power represents the power supplied from a DC power unit to the power amplifier. That is, the power added efficiency should be understood as a conversion efficiency from DC power to AC power. In the case of a cellular telephone using a battery as DC power source, it is desirable that the power added efficiency is sufficiently high to prevent premature exhaustion of the battery's stored power.
Radio (wireless) telephone transmitters, used in Japan, must comply with the NTT (Nippon Telegraph and Telephone Corporation) standard when they are operated in the analog modulation mode, or comply with the RCR (Research and Development Center For Radio S System) standard when they are operated in the digital modulation mode. The NTT standard stipulates the Frequency Division Multiple Access (FDMA) system. The RCR standard, under the requirements of Article STD-27B, stipulates the Time Division Multiple Access (TDMA) system. Both the FDMA and TDMA systems require a power amplifier having the same AC output power of 1.5 W.
FIG. 6 is a graph showing a typical example of input/output characteristics and power added efficiency in accordance with a conventional power amplifier of a dual-mode transmitter produced is for use in Japan. This power amplifier includes a single power amplification circuit to which a DC bias voltage Vdd of 6 V is supplied. The power added efficiency in a 1.5 W output condition, as shown in FIG. 6, is approximately 40% in each mode of FDMA (i.e. analog modulation) and TDMA (i.e. digital modulation). In short, this conventional technology causes a problem in forcing the power amplifier to sacrifice the power added efficiency in the FDMA mode in order to realize high linearity in the TDMA mode.
On the other hand, in the United States, there is the TIA (Telecommunications Industry Association) standard applied to the dual-mode radio telephone transmitters. The TIA standard, under the requirements of Article IS-95, stipulates the FDMA system as an access system for the analog modulation mode and the Code Division Multiple Access (CDMA) system as an access system for the digital modulation mode. The FDMA system requires an AC output power of 1.5 W, while the CDMA system requires an AC output power of only 0.5 W.
FIG. 7 is a graph showing an example of input/output characteristics and power added efficiency in accordance with a conventional power amplifier of a dual-mode transmitter produced For use in the Unites States. This power amplifier includes a single power amplification circuit to which a DC bias voltage Vdd of 4.8 V is supplied. The power added efficiency in the FDMA mode is approximately 60% in a 1.5 W output condition, while the power added efficiency in the CDMA mode is approximately 80% in a 0.5 W output condition. In short, this conventional technology causes a problem in forcing the power amplifier to sacrifice the power added efficiency in the CDMA mode in order to realize high efficiency in the FDMA mode.
As explained in the foregoing description, the conventional single-power-amplification-circuit type power amplifier for a transmitter is subject to the problem that, if it is produced for use in Japan, the power added efficiency is worsened in the analog modulation (FDMA) mode or the other problem that, if it is produced for use in the United States, the power added efficiency is worsened in the digital modulation (CDMA) mode. In short, there is an essential problem in that applying either set of standards the power added efficiency is worsened in either the analog or digital modulation modes.